The present invention relates generally to filters for use inside blood vessels. More particularly, the present invention relates to thrombus filters which can be securely affixed at a selected location in the vascular system and removed when no longer required.
There are a number of situations in the practice of medicine when it becomes desirable for a physician to place a filter in the vascular system of a patient. One of the most common applications for vascular filters is the treatment of Deep Venous Thrombosis (DVT). Deep Venous Thrombosis patients experience clotting of blood in the large veins of the lower portions of the body. These patients are constantly at risk of a clot breaking free and traveling via the inferior vena cava to the heart and lungs. This process is known as pulmonary embolization. Pulmonary embolization can frequently be fatal, for example when a large blood clot interferes with the life-sustaining pumping action of the heart. If a blood clot passes through the heart it will be pumped into the lungs and may cause a blockage in the pulmonary arteries. A blockage of this type in the lungs will interfere with the oxygenation of the blood causing shock or death.
Pulmonary embolization may be successfully prevented by the appropriate placement of a thrombus filter in the vascular system of a patient""s body. Placement of the filter may be accomplished by performing a laparotomy with the patient under general anesthesia. However, intravenous insertion is often the preferred method of placing a thrombus filter in a patient""s vascular system.
Intravenous insertion of a thrombus filter is less invasive and it requires only a local anesthetic. In this procedure, the thrombus filter is collapsed within a delivery catheter. The delivery catheter is introduced into the patient""s vascular system at a point which is convenient to the physician. The delivery catheter is then fed further into the vascular system until it reaches a desirable location for filter placement. The thrombus filter is then released into the blood vessel from the delivery catheter.
In the treatment of Deep Venous Thrombosis, a thrombus filter is placed in the inferior vena cava of a patient. The inferior vena cava is a large vessel which returns blood to the heart from the lower part of the body. The inferior vena cava may be accessed through the patient""s femoral vein.
Thrombus filters may be placed in other locations when treating other conditions. For example, if blood clots are expected to approach the heart and lungs from the upper portion of the body, a thrombus filter may be positioned in the superior vena cava. The superior vena cava is a large vessel which returns blood to the heart from the upper part of the body. The superior vena cava may by accessed through the jugular vein, located in the patient""s neck.
Once placed inside a blood vessel, a thrombus filter acts to catch and hold blood clots. The flow of blood around the captured clots allows the body""s lysing process to dissolve the clots.
The walls of the blood vessels are lined with a thin inner membrane or intima. When the anchor portions of a thrombus filter puncture this inner membrane the body responds to a puncture of the intima with a process known in the art as neointimal hyperplasia. As a result, the punctured area of inner membrane is overgrown with a number of new cells. The anchor portions of the thrombus filter are typically encapsulated with new cell growth (neointimal hyperplasia). Because the portions of the filter contacting the blood vessel wall become fixed in this way, it is impractical to remove many prior art filters percutaneously after they have been in place for more than two weeks.
There are a number of situations in which it may be desirable for a physician to remove a thrombus filter. If the physician determines that more effective filtering would occur with a thrombus filter in a different position, the physician may remove the original filter from its present positions and deploy a new filter in a new position. If the physician determines that the risk of blood clots forming is no longer present, it may be desirable to remove the thrombus filter completely. Thrombus filters are often used in conjunction with anticoagulation drugs. At some point, the physician may desire to discontinue the use of anticoagulation drugs. The physician may also want to remove the thrombus filter in conjunction with discontinuing the anticoagulation drugs. The removal of the thrombus filter from the patient eliminates any possibility that a compete occlusion will occur at the thrombus filter site. The removal of the thrombus filter also eliminates any possibility that the thrombus filter will become loose and migrate within the blood vessel. A loose thrombus filter is undesirable because it may migrate to a dangerous or life threatening position.
The present invention pertains to a thrombus filter and a method of removing a filter using minimally invasive methods avoiding complications due to neointimal encapsulation of anchor portions of the filter. The thrombus filter includes a body member and a plurality of elongated struts. Each strut has a joined end and a free end. The joined end of each strut is fixably attached to the body member. The struts radiate outwardly from the body member such that the thrombus filter is generally conical in shape. When the thrombus filter is deployed inside a blood vessel, the free ends of the struts engage the blood vessel wall. The body member of the thrombus filter is held in a position proximate the center of the blood vessel by the plurality of struts which engage the blood vessel walls with opposing force vectors.
When the thrombus filter is disposed in a blood vessel, the conical formation of struts acts to trap or capture blood clots. The generally conical shape of the formation of struts serves to urge captured blood clots toward the center of the blood flow. The flow of blood around the captured clots allows the body""s natural lysing process to dissolve the clots.
To assure firm attachment of the thrombus filter to the blood vessel walls, anchor portions may be formed at the free ends of the struts. These anchor portions typically include one or more bends and one or more sharp points. A weakened portion is disposed proximate the free end of each strut. The weakened portion of each strut may include notches, grooves, holes and the like.
When removal of the thrombus filter is desired, a removal catheter with a lumen and a distal end is disposed inside the blood vessel. The removal catheter enters the patient""s vascular system at a point which is readily accessible to the physician. Once in the vascular system, the removal catheter is urged forward until the distal end of the catheter is proximate the thrombus filter. The distal end of the removal catheter is then urged forward so that the body member of the thrombus filter is disposed inside the lumen of the removal catheter. A force is applied to the thrombus filter urging the body member further into the lumen of the removal catheter. The magnitude of this force is sufficient to break the struts of the thrombus filter at the weakened portions proximate the free ends of the struts. When the struts are broken, the thrombus filter may be pulled freely into the lumen of the removal catheter. Removal of the thrombus filter from the body of the patient then becomes a matter of simply withdrawing the removal catheter from the blood vessel. The anchor members of the thrombus filter remain attached to the walls of the blood vessel by encapsulating cell growth due to neointimal hyperplasia.
An alternate method of removal involves repeatedly deflecting the struts with a force which is not of sufficient magnitude to break the struts of the thrombus filter at the outset. However, the repeated deflection of the struts causes fatigue cracks to grow at the weakened portions. As described above, the cross sectional area of each strut is reduced at a weakened portion including slots, holes, and the like. The cross sectional area of the struts is further reduced by fatigue cracking due to repeated deflection of the struts. After multiple deflections, the cross sectional area of the struts, at the weakened areas will be small enough that a small force alone is sufficient to break the struts at the weakened areas.